There are still contradictory customs around this enough that it is standard practice to warn exchange students from Europe that if they finish absolutely everything on their plate that this is a signal in many American homes that you should be served more. This can lead to some real discomfort as the student tries to eat everything they are given which leads to being given more and more.

So at the same time it is considered poor taste to take more than you can eat, it is also considered poor form to offer a guest anything less than more than they can eat. This also shows up when people rate restaurants by the serving size.